Anaglyphs are the earliest of several systems used to bring the illusion of depth to two dimensional images. Stereopsis (from stereo- meaning “solid” or “three-dimensional” (3D), and opsis meaning view or sight) is the process in visual perception leading to the perception of depth from the two slightly different projections of the world onto the retinas of the two eyes. The image differences arise from the eyes' different positions in the head (the inter-pupillary distance). The displacement of objects resulting from the different points of view of the two eyes is called binocular disparity, horizontal disparity, or retinal disparity. The image differences help enable 3D image perception due to parallax, or the apparent displacement or difference in the apparent position of an object viewed along two different lines of sight.
Anaglyph images can be viewed directly on a display or as printed images. In general, an anaglyph is a stereoscopic image generally consisting of two distinctly colored, and preferably, complementary colored, prints or drawings that comprise superimposed left and right eye images. In particular, each image is color coded and viewed through a suitably colored filter, so each eye sees only its own image. For example, when a red line on white paper is viewed through a red filter, it disappears within the white surround, while a green or cyan line viewed through the same filter is dark and clearly visible. Typically the inks used are usually roughly complementary (red and green, orange and green, or red and cyan), while the filters are either red and green or red and cyan. Although the two views or source images are complete and show all or nearly all features of the scene, the two images of the scene are captured at positions roughly similar to the relative positions of the left and right eyes from human vision. In appearance, content in one of the image of an anaglyph image appears to be shifted relative to the other image. These shifts, the binocular disparity, exhibit the color fringing or ghosting seen in an anaglyph image looked that is examined as a two-dimensional (2D) image. In the anaglyph process, an observer wears glasses with two different colored lenses to filter the different images to the correct eye and produce a 3D effect. That is, the disparity or horizontal displacement between objects in the cyan and red images results in stereoscopic sensation of depth.
Traditionally, a stereo pair is made by capturing a scene with two horizontally displaced cameras, each of which images through an optical filter to capture the color separation images. With modem image processing software, stereo images can also be created by post-capture processing images captured by a single camera. To clarify traditional camera-based capture, FIG. 1A depicts an illustrative left eye source image 60 of a boy and girl captured from a left camera position (shown as left camera position 90 in FIG. 1D) and a right eye source image 65 of the same scene captured from the right camera position 95 (again FIG. 1D) is shown as FIG. 1B. It should be understood that the distance between the left eye camera position 90 and the right eye camera position 95 is typically equivalent to the human interocular distance but can be a greater or smaller distance depending on the sense of perspective that is desired. These source images are composed to produce an anaglyph image 50 (illustrated as FIG. 1C) with a red image channel 62 equal to the red channel of FIG. 1A, and with green and blue image channels 64 equal to the green and blue channels (respectively) of FIG. 1B. As shown in FIG. 1C, the anaglyph image 50 appears as two overlapped but offset images, whose offsets are exaggerated for clarity.
When an anaglyph is viewed, a color filter over each eye only transmits the image component suitable for that eye and the brain of an observer fuses the images and interprets the result in three dimensions (3D). Typically, observers wear stereo glasses 40 (e.g. 3D glasses or multi-view glasses), shown in FIG. 2A, which use filters (usually inexpensive gel color filters) such as a left eye filter 46 and right eye filter 48 to provide different color filtering for each eye. For example, one eye's filter blocks first portions of the visible spectrum and transmits second portions, while the second eye's filter is generally inverted to transmit the first portions and block the second portions. Usually the main subject is in the center, while the foreground and background appear to have been shifted laterally relative to the other view and in opposite directions. Most commonly, anaglyphs are viewed with colored stereo glasses 40, as exemplified by those shown in FIG. 2A, having a red left eye filter 46 and a cyan right eye filter 48, although various other filter combinations, including blue or green filters, are used.
FIGS. 2B and 2C depict exemplary transmission spectra for a set of available red filters 43 that can be used as left eye filter 46 and exemplary transmission spectra for a set of available cyan filters 44 used in right eye filter 48 of stereo glasses 40. The red filters 43 substantially transmit red light, while substantially blocking blue and green light, but varying degrees depending on the filter. Thus, the red left eye filter 46 shows red image areas or highlights from white image areas as bright image areas. On the other hand, image areas that are black or cyan (or blue or green) all appear as dark (grey to black) through the red left eye filter 46. Similarly, cyan filters 44 (FIG. 2C) provide a view through right eye filter 48 that sees cyan light but not red light. When the image content includes left and right source images 60 and 65, color coded to work with anaglyph stereo glasses 40, a three dimensional image providing a sense of depth can be perceived by an observer wearing glasses 40.
In particular, as shown in FIG. 2D, left and right source images 60 and 65 are combined to provide an anaglyph image 50. The anaglyph image 50 provides image difference coded images to an observer wearing stereo glasses 40 having filters 42. Nominally the image differences are color coded and the filter 42 are color filters, such as red filter 43 and cyan filter 44, although some stereo glasses 40 have filters 42 that are polarization filters. The observer (not shown), who benefits from both mental processing and experiential expectations concerning depth perception and perspective, can then cognitively fuse and recognize a perceived 3D image 20 within their brain. It should be understood that the observer can also be viewing a printed dynamic anaglyph image 420 of the type of the present invention.
As a 3D visualization technology, anaglyphs are simple. Additionally, their size is unlimited and they are a convenient and readily available way to produce printed images that can be perceived by most people as a stereo image. The principle of anaglyphs was first illustrated using blue and red lines drawn on a black field, with observers wearing red and blue glasses to perceive the effect. In 1895, Louis Ducas du Hauron invented the anaglyph stereoscopic print (U.S. Pat. No. 544,666), in which stereoscopic photographs were reproduced as anaglyph prints on paper viewable by an observer wearing color filter glasses. This was accomplished by a process of printing the two negatives, one in blue (or green) and the other in red, on the same paper to form the stereoscopic photograph.
Although anaglyphs are readily produced, they are not without issues, which include ghosting (where a color from the left image leaks to the right eye (and vice versa)), left eye and right image mis-registration that affects image fusion, and retinal rivalry (the two images have unequal luminance, and one eye dominates or there is perceived flicker). For example, blue image ghosting can occur, resulting in a violet or purple image instead of a black and white one. As an attempt to resolve such problems, prior art U.S. Pat. No. 2,135,197 by J. A. Norling provides an approach for providing anaglyph stereoscopic prints using a blue-magenta filter pair, in which secondary yellow filtering is used to eliminate the blue image ghosting. As another example, the paper “Choice of Inks for Printed Anaglyphs”, by W. Cox, published in Applied Optics, Vol. 16, pp. 2586 (1977), proposes using to reduce the ghosting of the green image through the green filter, by using a green fluorescent ink instead of a normal process ink that has significant red spectral response, and thus can leak red light through the green transmitting filter. The green fluorescent dye boosted the green luminance, allowing the green passband filtering to be narrowed.
As previously noted, with the early anaglyph technologies, observers viewed tinted black and white or monochrome images, though they wore red-green or red-blue pair color filter glasses. Of course, it was desirable to provide colored anaglyphic pictures. U.S. Pat. No. 4,620,770 by H. Wexler, entitled: “Multi-Colored Anaglyphs”, provided one approach. In the '770 patent, an anaglyph is created using two distinctly or approximately complementary colored pictures, such as a red and blue color pair. The elements of each picture appear in outline only, with shifted outlines being rendered in one of the two chosen colors. The two colored pictures are prepared on a white, off-white or lightly colored background. Whereas, the interior portions of the elements of one of the two pictures are colored according to the whim or aesthetics of the designer.
With the common red and cyan (green+blue) filter pair, color images can be seen, however, color reproduction is compromised, as each eye is receiving only a portion of the visible color spectrum, and the color quality is poor. Although a truly full-color stereoscopic image is not considered achievable with anaglyphs, a properly constructed anaglyph using complimentary colors can approximate a full-color image. Various approaches have been developed to improve the color quality, often using color filter combinations other than typical red-cyan or red-green filter pairs.
As a particular example for enhanced color anaglyphs, the “Anachrome” approach is a variant of the red-cyan filter pair in which the left eye has a dark red filter, while the right eye has a cyan filter that leaks 2%) some red light. This color channel pairing provides better color perception results, as the images can show red hues better than red-cyan filter pairing. This assigns two-eyed “redness cues” to objects and details, such as lip color and red clothing, that are fused in the brain. However, in the =chrome approach, care must be taken to closely overlay the red areas into near-perfect registration, or “ghosting” can occur. “Mirachrome” is another alternate red-cyan system, similar to anachrome, but with the addition of a weak positive correction lens on the red channel to compensate for the chromatic aberration of eyes. Related U.S. Pat. No. 6,561,646 by A. Silliphant provides stereo filter glasses in which one lens has an optical corrector. In contrast, the “trioscopic” approach uses green and magenta filtering instead of red-cyan filtering, to provide better color and less chromatic aberration than the anachrome approach. The ColorCode 3D approach, described in U.S. Pat. No. 6,687,003 by S. Sorensen et al., also uses an alternate color filter pair, amber and dark blue, to provide improved color rendering, although the images can be dark.
While the color appearance of an anaglyph image can be enhanced by using an alternate color coding scheme to the traditional red-cyan channel pairing, crosstalk can be differentially impacted. The paper, “Comparing levels of crosstalk with red/cyan, blue/yellow, and green/magenta anaglyph 3D glasses”, by A. Woods et al., published in SPIE Vol. 7254, pp. 75240Q-1-12 (2010) examines this issue in detail for stereo glasses having a range of different spectral filter profiles (including gel, inkjet printed, and dichroic filters) which are used to view anaglyphs seen electronically on different electronic displays with different emission spectra (plasma, CRT, and LCD). While the results do not apply directly to printed content, it can be concluded that different color coding schemes can work either significantly better or worse for crosstalk depending on the pigment and stereo glasses filter spectra used.
It is noted that as image understanding and computational abilities have improved, it is no longer necessary to capture a stereo image pair with two cameras in order to produce anaglyph or other stereo images. Stereo images can be effectively derived from still or video images captured using a single camera. Stereo images can also be generated synthetically to create realistic views or to create stylized or surreal views for use in virtual worlds, animation, or video games. Whether anaglyphs are created from real images or synthetically, various techniques can be applied to improve 3D perception and minimize artifacts such as retinal rivalry or crosstalk. For example, U.S. Pat. No. 6,389,236 by O. Western, entitled “Anaglyph and Method”, provides for improved anaglyphs with reduced retinal rivalry in which the convergences angle and parallax are optimized by a ray mapping method.
One problem with prior art anaglyphs images is that once printed, they can only be properly viewed by observers wearing the appropriate stereo glasses 40. In particular, when an anaglyph is viewed as a two dimensional (2D) image, the offset and overlapped color images contain pronounced color fringing (colored borders of objects) which is often confusing and distracting to the viewer. For example, on a 4″×6″ image, the color fringes can extend for several millimeters (mm) about the edges of the image content. This problem has been addressed by altering the position, location, or color of the color encoded disparity data. For example, in a recent paper, “A Perceptual Model for Disparity”, by P. Didyk et al., ACM Transactions on Graphics, Vol. 30(4) (Proceedings SIGGRAPH 2011), a perceptual model of binocular disparity was provided which includes a stereo image difference metric to compare a stereo image to an alternative stereo image and to estimate the magnitude of the perceived disparity change. As an example, the disparity model is used to create “backward compatible” anaglyph images that appear as successful 2D images, but also as successful 3D images when colored glasses are worn. Starting from a pair of stereo images, disparity is compressed or flattened by removing low frequency content, which improves backward compatibility. However, at the same time, the change in the stereo image difference metric is monitored to ensure that at least a specified minimum of perceived disparity remains. Didyk et al. suggest that the loss of the low frequency image difference data causes disparity decay that is mostly invisible, while the disparity discontinuity and apparent stereo depth are effectively retained. When the resulting images are examined, the backward compatibility is largely successful, as visible artifacts are greatly reduced when the image is viewed as a 2D image without stereo glasses. However, for such methods, the perception of stereo or 3D image quality is noticeably degraded. Additionally, relative to the original anaglyph stereo image, some depth information is lost.
Alternately, in U.S. Patent Publication 20090278919, entitled “High Fidelity Printed Anaglyphs and Viewing Filters”, and filed by M. Ramstad, proposes an approach for a printed anaglyph that has an acceptable image appearance for both 2D and 3D viewing. In particular, the '919 publication proposes that the anaglyph can be printed with conventional inks, as well as UV stimulated, visibly fluorescing inks. One eye's image is viewed through a first viewing filter which substantially transmits red, green and blue components of the first image and blocks the fluorescent color. The second eye's image is viewed through a second viewing filter which substantially transmits the fluorescent color and blocks the other regions of the visible spectra. However, in normal lighting, the self emissive fluorescent inks may provide the viewers with a halo effect along the edges of the printed content, rather than a 3D effect, as the fluorescing inks glows, and provides a much brighter luminance than the surrounding content printed with non-fluoreses conventional inks. The intensity of the ambient visible light illumination may have to be raised to compensate. Additionally, anaglyph image disparities are normally provided to both eyes, but the '919 publication only provides highlighting to one eye, which would limit the effectiveness of these images.
Therefore, it would be desirable to provide printed anaglyph images that have both a two dimensional viewing state and a three dimensional viewing state without incurring significant loss in either 2D or 3D image quality. In particular, it would desirable if the image fringing or disparities at the image content edges that enable the 3D view are minimally visible when the printed image is in the 2D image viewing state. Moreover, it would be desirable if such printed anaglyph images can be readily switched between the 2D and 3D image states in a repeatable and controlled manner.